chore(CPlusPlus): added math factorial (#499)

algorithms/CPlusPlus/Maths/factorial.cpp

@@ -0,0 +1,42 @@
+/*
+Description: A program to calculate factorial of a number.
+A factorial of number 4 is calculated as:
+4 X 3 X 2 X 1 = 24
+
+Approach: Calculating factorial using for loop.
+Declaring the f varialbe to 1 (not initialising it to zero because any number multiplied by 0 will be 0)
+Multiplying the f variable to 1,2,3...n and storing it in the f varialbe.
+The same factorial can be calculated using while loop, recursion.
+
+Time Complexity: O(number)
+*/
+
+#include <iostream>
+using namespace std;
+
+//function starts
+long factorial (long n){
+  long f=1;
+  for(long i=1;i<=n;i++){
+    f=f*i;
+  }
+  return f;
+}
+
+//main starts
+int main() {
+  cout << "Enter a number: \n";
+  long n;
+  cin>>n;
+  cout<<"factorial is: "<<factorial(n);
+  return 0;
+}
+
+/*
+Input: 
+Enter a number: 
+3
+
+Output:
+factorial is: 6
+*/

algorithms/CPlusPlus/Maths/missing-number.cpp

@@ -0,0 +1,53 @@
+/*
+Description: Given an vector v containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the vector.
+
+Approach: Using for loop from 0 to n and searching if the value is present in the vector or not using find() function.
+If a certain value is not present, the function will return it.
+*/
+
+#include <iostream>
+#include <vector>
+#include <algorithm>
+using namespace std;
+
+//function starts
+int missingNumber(vector<int> &v){
+  int num;
+  for(int i=0;i<=v.size();i++){
+    //checking if i is present or not in the vector
+    //if not present,it will store the value of i in the num variable and breaking it
+    if(find(v.begin(),v.end(),i)==v.end()){
+       num=i;
+       break;
+    }
+  }
+  return num;
+}
+
+//main starts
+int main() {
+cout << "Enter number of elements:\n";
+int n;
+cin>>n;
+vector<int> v(n);
+cout<<"Enter any "<<n<<" elements (From 0 to "<<n<<") \n";
+for(int i=0;i<n;i++){
+  cin>>v[i];
+}
+cout<<"\nMissing number is: "<<missingNumber(v);
+return 0;
+}
+
+/*
+Input:
+Enter number of elements:
+4
+Enter any 4 elements (From 0 to 4) 
+0
+1
+2
+3
+
+Output:
+Missing number is: 4
+*/

algorithms/CPlusPlus/README.md

@@ -126,6 +126,8 @@
 5. [Armstrong Number](Maths/armstrong.cpp)
 6. [Palindrome](Maths/palindrome.cpp)
 7. [Reverse digit of a number](Maths/reverse-digits.cpp)
+8. [Missing number](Maths/missing-number.cpp)
+9. [Factorial of a number](Maths/factorial.cpp)
 
 # Recursion